Overview
This lecture explains how neurons generate and transmit electrical signals (action potentials), covering the basics of electrochemistry in nerve cells and the process of communication along the axon.
Neuronal Communication Basics
- Neurons communicate using identical electrical impulses called action potentials.
- The frequency of action potentials varies to convey different information.
- The brain interprets impulse patterns to distinguish different sensations or actions.
Electricity in the Body
- The body is electrically neutral overall but maintains local charge differences across cell membranes.
- Cell membranes act as barriers to separate positive and negative charges, creating electrical potential.
- Voltage is the potential energy from separated charges; in cells, it's measured in millivolts.
- Current is the flow of charged ions across membranes.
- Resistance refers to how much membranes inhibit ion flow.
Resting Membrane Potential and Ion Gradients
- Resting membrane potential of a neuron is about -70 mV (more negative inside than outside).
- Sodium ions (Na⁺) are concentrated outside, while potassium ions (K⁺) and negatively charged proteins are inside.
- The sodium-potassium pump maintains this gradient by moving 3 Na⁺ out and 2 K⁺ in.
- This polarization creates an electrochemical gradient across the membrane.
Ion Channels and Membrane Potentials
- Ion channels let ions cross membranes when open; types include voltage-gated, ligand-gated, and mechanically gated channels.
- Voltage-gated channels open at specific membrane potentials (e.g., sodium channels at -55 mV).
- Stimuli cause graded potentials if small, but need to cross a threshold (-55 mV) for an action potential.
Action Potential Process
- At rest, all channels are closed and neuron is polarized.
- A strong stimulus opens sodium channels, depolarizing the membrane.
- If threshold is exceeded, many sodium channels open, causing rapid depolarization up to +40 mV.
- Action potential propagates as each segment triggers the next.
- Repolarization occurs as potassium channels open, letting K⁺ out, sometimes causing hyperpolarization (< -70 mV).
- Sodium-potassium pump restores resting potential.
- The refractory period prevents new impulses until the neuron resets.
Signal Frequency, Speed, and Myelination
- Action potential strength is always the same; intensity is coded by frequency of impulses.
- High-frequency signals mean stronger or more urgent stimuli.
- Myelinated axons conduct impulses faster via saltatory conduction, jumping between Nodes of Ranvier.
Key Terms & Definitions
- Action Potential — a rapid electrical impulse moving down a neuron’s axon.
- Resting Membrane Potential — the voltage difference across the neuron membrane at rest (about -70 mV).
- Sodium-Potassium Pump — protein that exchanges Na⁺ and K⁺ to maintain charge gradients.
- Ion Channel — protein that allows ions to move across cell membranes.
- Depolarization — loss of membrane polarization (cell becomes more positive inside).
- Repolarization — return to resting membrane potential after depolarization.
- Hyperpolarization — membrane potential becomes more negative than at rest.
- Refractory Period — time during which a neuron cannot fire another action potential.
- Myelin Sheath — insulating layer around some axons speeding up impulse transmission.
- Saltatory Conduction — process where impulses jump between gaps in myelin (Nodes of Ranvier).
Action Items / Next Steps
- Review the stages of the action potential and key terms.
- Prepare to learn about how action potentials transfer between neurons in the next lesson.